Polishing a workpiece to produce a mirror-like, defect-free surface has applications in many fields of endeavor. Such polishing processes have become extremely important and widespread, for example, in the fabrication of semiconductor devices. The critical step of polishing a semiconductive wafer or substrate is required at a number of different stages along the varied processes employed to fabricate semiconductor devices.
The manufacture of integrated circuits generally involves an elaborate system of fabricating semiconductor devices on a wafer or substrate and connecting the devices together. The devices are connected by a process generally referred to as metalization, in which connecting lines of metal, often aluminum, are applied by vacuum deposition or other suitable processes.
The performance level of semiconductor devices employing a conventional single metal layer connecting the devices is fast becoming unsuitable. Modem, high performance devices utilize multilevel metal interconnections. Multilevel connections may be constructed by depositing a dielectric or insulating layer over a first metal layer, etching via holes throughout the dielectric layer, and then depositing a second metal layer which fill the via holes to connect with the first metal layer. These devices offer higher device density and shortened interconnection lengths between the devices.
Since each of these metal and dielectric layers have an appreciable thickness, the wafer substrate is left with a non-planar topography as the various layers are patterned on top of one another. This type of non-planarity is often unacceptable in high density devices because the depth of field of the lithographic equipment that is used to print the smaller line width circuits on the wafer does not have a depth of focus sufficient to compensate for even small variations in wafer planarity.
In addition to the non-planarity caused by the fabricated device patterns, in-process wafer polishing, or planarization, must account for variations in overall wafer flatness as well. During the fabrication process, for example, the wafers may become bowed or warped.
In process polishing equipment, therefore, requires the specialized ability to achieve global, uniformly planar wafer surfaces in spite of these topographical wafer defects and variations. Chemical-mechanical polishing has gained wide acceptance as an effective means of achieving the global wafer surface planarity required by advanced devices employing multilayer metalization.
FIG. 1 shows a partial cross section of a typical prior art chemical-mechanical polishing arrangement. A typical device includes a tooling head having a generally circular pressure plate or carrier platen 1 that supports a single substrate or wafer 3. A carrier film 2 may be interposed between the carrier platen 1 and the wafer 3 to partially accommodate wafer thickness variations. The tooling head is equipped with means to provide a downward force, urging the wafer 3 against a polishing media 5 (typically a circular pad), onto which is fed a polishing fluid 7. The polishing media is supported by the polishing platen 6. The polishing fluid 7 may comprise a colloidal suspension of an abrasive and may also comprise of a chemically reactive solution. A containment ring 4 generally surrounds the wafer to prevent it from slipping off the carrier platen 1 during polishing.
Typically, movement of the wafer relative to the pad, in the presence of the chemically reactive and/or abrasive polishing fluid and under pressure imparted by the tooling head, imparts a combination of chemical and mechanical forces to the wafer 3, the net effect of which is global planarization of the wafer surface. Generally, the polishing platen 6 is rotatable as is the carrier platen 1. In a typical polishing apparatus, relative movement of the wafer relative to the pad is accomplished by rotating the polishing platen 6, the carrier platen 1, or both.
Because the manufacturing plant required to produce semiconductor devices is very costly, it is important that each piece of semiconductor process equipment installed in the fabrication line make economical use of the time required for its particular process and the physical space required for use. For this reason, there is constant pressure to improve total process throughput and reduce the amount of floorspace required for semiconductor process equipment.
In this regard, polishing machines used in the semiconductor device fabrication process are not optimized. Current chemical-mechanical polishing machines do not have the ability to deliver substantially uninterrupted polishing media and have difficulty in uniformly conditioning the polishing media for continued use after a certain amount of polishing has been performed.
Rotating platen machines typically install a circular polishing pad and use it until the pad fails to obtain acceptable results because the pad becomes worn or becomes glazed with impacted polishing fluid and polishing particulate. At that time it is required to interrupt the polishing process and change the polishing pad.
Some polishing machines have employed a conditioning device, such as a spinning head, to condition the pad. The purpose of such conditioning is to create and revitalize the structure on the polishing media that retains the slurry dispersion for the polish process. Conditioning also serves to liberate and remove material impacted into the pad in the course of polish processing. Specific applications of conditioning may serve to planarize the polishing pad as well as cut or form a desired pattern into the polishing surface. Such a pattern is useful, for example, to facilitate uniform slurry distribution over the pad.
In current machines, conditioning is generally performed in the same planar area as the processing area. Typically, when the platen is rotating, a smaller spinning head is moved across the radius of the platen. Because of the inherent surface velocity differentials associated with the spinning head and the rotating platen, it is very difficult to ensure that a constant relative velocity between the rotating platen and the spinning head was accomplished at every point during conditioning. Such a constant relative velocity is required to ensure equal and uniform conditioning.
In addition, conditioning defects are often created as a result of misalignment of the surface of the spinning head to the plane of the polishing media. Even when suitably aligned, once polishing begins the spinning head is subject to substantial and varying friction forces at the conditioning surface. These forces tend to adversely affect the alignment of the head causing an edge of the spinning head to dig into the pad. The inability to provide uniform, defect free conditioning of the polishing pad surface inevitably causes a corresponding degradation in the polish processing results. Furthermore, while these types of conditioning may extend the polishing pad life somewhat, it is still required to install a new pad at a higher than desired frequency.
Another source of concern for the creation of wafer defects as a result of the conditioning is in the advent of conditioning particle(s), or conditioning elements becoming liberated from the conditioning device used, wherein the liberated conditioning elements (i.e. diamond chip) can become imbedded in the polishing media. The imbedded conditioning elements are a primary source of scratch defects found in wafers in the course of process polishing. Given the appearance of wafer scratch defects, the process requires immediate shutdown for replacement of the polishing pad media.
Another area that has not been optimized on current polishing machines is the control and maintenance of polishing fluids. Control and containment of the polishing fluid is important for many reasons. Typically, the polishing fluid is comprised to provide not only the abrasive particulate for mechanical polishing, but also to be chemically reactive with the wafer surface that is to be polished. It is important, therefore, that there exists on the polishing media an adequate amount of polishing fluid so that no areas of the wafer suffer any deficiency of polishing fluid as the wafer is moved relative to the polishing media. For example, if parts of the wafer were to suffer from a deficiency of polishing fluid, it would accordingly have a removal rate different from the areas of the wafer that were adequately supplied.
Another aspect of the importance of polishing fluid control is the desire to make the most efficient use of the polishing fluid. As mentioned above, polishing fluid applied to the polishing media generally spills or washes over the edge of the polishing platen at some point during the polishing process on current polishing machines. This polishing fluid is not readily reusable or recoverable and must be replaced by an adequate amount of new polishing fluid. It should be apparent that allowing the polishing fluid to exit the polishing media too quickly will result in an increased rate of polishing fluid usage over time thus lowering efficiency and increasing costs. Allowing the polishing fluid to have an increased resident time on the polishing media before exit, on the other hand, allows for full use of the polishing fluid before it becomes chemically unreactive or contaminated.
Still another aspect of the importance of polishing fluid control and containment is the need to maintain the polishing fluid in a clean, controlled space. Prior art devices employing a spinning platen and wafer carrier do not allow for adequate control of the polishing fluid as it exits the rotating polishing media. Polishing fluid is disadvantageously allowed to reach places where it can cause cleanliness problems, equipment reliability problems and, perhaps more importantly, polishing defects on the wafer itself.
The problems associated with allowing polishing fluid to reach places other that the front side of the polishing media is due to the fact that the polishing fluids used may dry to form a hard substance. These dried deposits are not only difficult to clean but may also disadvantageously flake off into unwanted places within the polishing environment. If the polishing fluid is allowed to reach the back side of the polishing media, it may form a raised deposit between the polishing platen and the polishing media resulting in polish defects in the wafer due to the increased removal rate in the vicinity of the raised deposit. Even when wet, the polishing fluids may tend to attack the pressure sensitive adhesives used in the construction of many of the composite polishing media, causing pad delamination and polishing failure.
Devices known in the art have not provided a complete solution to these slurry containment problems. For example, some polishing devices employ a vertical ring or dam structure around the periphery of a rotating polishing platen. U.S. Pat. Nos. 4,910,155 and 5,299,393 disclose the use of a separate dam structure designed to be placed around the circular periphery of the polishing table to prevent leakage of liquid polishing fluid from the polishing table. The dam allows a pool of polishing fluid to be formed to a depth of about one-quarter inch. With this arrangement, however, the polishing fluid periodically becomes contaminated and chemically unreactive requiring it to be washed off the polish table and replaced with a fresh batch of polishing fluid.
Other polishing devices provide for a more slanted ring feature around the periphery of the circular polishing table. U.S. Pat. No. 5,398,459 discloses a turntable ring positioned radially outwardly of the polishing media to help the abrasive polishing fluid from being scattered around during polishing. U.S. Pat. No. 5,384,986 discloses a similar structure designed to help prevent a protective fluid, such as water, from flowing radially off the pad to prevent the pad from drying out when not polishing. The slant and incline height of the surface is selected, however, to allow fluid on the polishing platen, or turntable, to be scattered quickly outward away from the polishing platen when the turntable is rotated during polishing.
Still another device, disclosed in U.S. Pat. No. 5,516,400, attempts to create a dam or fluid hillock by forming an integral annular lip on the front surface of a top layer of a composite pad. An upper polishing pad is mounted to a lower pad and is sized so that an extreme outer edge portion of the upper pad extends beyond the lower pad. This outer edge portion is bent downwardly so that the polishing fluid that spills over the hillock is less able to wick back up to the lower pad to attack the adhesive and cause delamination.
While these devices tend to create a polishing fluid pool which may help to ensure the availability of an adequate amount of polishing fluid at the point of polishing, none of the devices are able to fully contain the polishing fluid from reaching undesirable locations and at the same time allow for continuous feed of new polishing fluid material for uninterrupted, continuous polishing.
It would be desirable to have an apparatus with the capability to deliver a continuous supply of polishing media without need for frequent operator intervention. It would also be desirable to have an apparatus with the ability to condition such a polishing media supply as needed as well as adequately contain the fluids used during processing. Such an apparatus would advantageously provide improvements in economy, processing throughput, reliability and amount of required floorspace for operation.